The present invention relates to a process for separating from each other aniline derivatives having a different number of C atoms bound to the N atom.
Aniline and N-alkylated aniline derivatives are of great importance in the synthesis of valuable intermediates. For example, N,N-dimethylaniline is an important starting material for the preparation of dyes of the triphenylmethane series (Michler's ketone, methyl violet, crystal violet and others). Monomethylaniline is, in addition to being used for the preparation of dyes, used mainly as an intermediate for pharmaceutical preparations.
The industrial synthesis of N-alkylated aniline derivatives from aniline and suitable alkylating agents generally gives mixtures of substances which are alkylated to different degrees and, in some cases, also still contain starting material. To separate the mixtures of compounds formed, complicated and expensive processes are necessary. For example, methylation of aniline in the presence of methanol gives a mixture of N-monomethylaniline and N,N-dimethylaniline and, in some cases, even aniline. Due to the small difference in boiling points of these liquids, a vacuum distillation achieves a satisfactory separation only at very high reflux ratios. If higher purities are required of the N-methylaniline, it must be, in addition, further treated with toluene sulphochloride or with phthalic anhydride (Ullmanns Enzyklopadie der technischen Chemie, (Ullmann's Encyclopaedia of Industrial Chemistry), 3rd edition, vol. 3 (1953), p. 652).
It was therefore desired to simplify those expensive and complicated separations.